The molecular nature of genetic mutations resulting in deficiencies of human LDH-A or LDH-B isozyme has been illustrated. The LDH-A deficiency is a latent heredity disorder of exertional myopathy. Female patients frequently complain of uterine stiffness during pregnancy, and delivery by Caesarean section is necessary. A new method based on PCR-amplified DNA was developed to detect the heterozygous carriers and homozygous deficient patients of human LDH-A gene. Rat cDNAs encoding lactate dehydrogenases B (heart) and C (testis) have been sequenced and their amino acid sequences deduced. Comparison of the DNA and protein sequences of LDH-A (muscle), LDH-B and LDH-C from human, mouse and rat shows different extents of variation among these three LDH genes. This information is of value to improve health care particularly of woman and it will also allow more accurate evaluation of genetic mutation events caused by environmental mutagens.